# What PSU to lower WHS watt consumption ?

• ### Question

• Is this even possible to choose a PSU that lowers the watt consumption of a computer ? Or is the watt consumption given by all the material you have in the computer (motherborad, CPU, hard disk, etc..) ann a PSU cannot change anything about it?

Thanks

Dag

Friday, November 16, 2007 3:09 PM

• Dag,

Directly, no. A PSU has no influence on how much power is dissipated by the rest of the items in the computer and so cannot influence the total consumption.

However, not all PSU's are the same. Some are more efficient than others and it's worthwhile checking the efficiency of the particular PSU you want to use. Some of them are now beginning to offer 80% plus versions, but you need to check at what point in the power curve they give their calculations; ie., if you buy a 500W unit but only run it at half load or so, is it as efficient as buying a 350W and running it harder.

HTH,

Colin

Friday, November 16, 2007 5:31 PM

### All replies

• Dag,

Directly, no. A PSU has no influence on how much power is dissipated by the rest of the items in the computer and so cannot influence the total consumption.

However, not all PSU's are the same. Some are more efficient than others and it's worthwhile checking the efficiency of the particular PSU you want to use. Some of them are now beginning to offer 80% plus versions, but you need to check at what point in the power curve they give their calculations; ie., if you buy a 500W unit but only run it at half load or so, is it as efficient as buying a 350W and running it harder.

HTH,

Colin

Friday, November 16, 2007 5:31 PM
• Colin is mostly right. A power supply doesn't affect the power consumption of other components. However, an 80+ power supply is at least 80% efficient across a wide range of power usage. It has to be 80% efficient, and deliver a power factor of .9 or greater, at 20% power, 50% power, and 100% power. An typical power supply today might be only 70% efficient at it's normal load. That means that if it's consuming 300 watts of power, it's only supplying 210 to the PC. The other 90 watts are turned into heat, which is quite a bit of waste.
Saturday, November 17, 2007 4:53 AM
• Interesting post,  I have just posted my specs in another thread, and will be interested to check on power usage of the system i have put together.

Saturday, November 17, 2007 5:24 PM
• Thanks a lot for theses informations.

Dag

Sunday, November 18, 2007 8:14 PM
• As has been touched on in this thread, here's the summary:

1) The power supply DOES consume some power all by itself. When people talk about an 80+ power supply, remember that the other 20- refers to power coming out of the wall but getting dissipated by the power supply without regard to the components you are powering with the supply. So the PSU most definitely influences total consumption of the system by its own consumption.

2) PSUs are not uniformly efficient under all loads. A PSU that's 85% efficient at 90% of its rated total output is unlikely to be that efficient at, say, 20% of its rated total ouput. This is why the race to get HIGH CAPACITY supplies may make marketing sense--like selling horsepower in a car, who'd want LESS?--but doesn't necessarily make energy sense. If you only need to draw 50 Watts, a 60 Watt rated supply that's, say, 85% efficient at 84% output makes a lot more sense than a 450W one that's, say, 60% efficient at 11% output. (The former is dissipating 9 Watts in the supply itself. The latter would be dissipating 33 Watts in the supply itself.) Unfortunately: 1) power supply marketing emphasizes higher outputs as universally better, 2) few components give end consumers enough data to do the engineering necessary to figure out what the system is really going to draw though some of the online estimating tools are pretty good, 3) specs for things like efficiency as a function of load are not readily available for most supplies, 4) efficiency will degrade over time due to aging effects, so this has to be taken into account, and 5) most electronic components will be more reliable when operated at some fraction of their rated capacity than at 100% of their rated capacity.

3) What you draw from the electric company and how fast your meter actually spins are not the same thing. This is a function of what is called power factor. IF you care about what you draw from the electric company (i.e., you want to keep your electric bill down and reduce emissions at the power plant), seek out a "power factor corrected" supply. These are common in Europe where they care about what the electric company has to produce (as opposed to just worrying about how much the consumer's meter spins) and have regulated power factor of devices. They are much less common in the US where we have done neither. But you can still find them if you go looking. I was a bit shocked to measure my WHS machine and find the power factor in the low 0.5s. (This was with the stock PSU that came with the case.)

Monday, November 19, 2007 7:25 PM